1. Field of the Invention
The present invention relates generally to unloaders for reciprocating gas compressors, and in particular to an unloader system that allows variable use of fixed or variable clearance volumes.
2. Description of the Related Art
Gas compressors are well known and various types have been utilized to meet the requirements of particular applications. For example, natural gas transmission through pipelines is often accomplished with large, reciprocating compressors driven by internal combustion engines at pumping stations located along the pipeline routes.
In natural gas transmission, the internal combustion engines which drive the compressors are often fueled by natural gas taken directly from the pipeline. Thus, the fuel consumed by the engines driving the compressors reduces the overall operating efficiency since the amount of gas delivered is reduced by amounts consumed in the transmission or pumping process.
Efficient operation of natural gas compressors typically involves the use of a computerized control system for controlling the air/fuel mixture, rotational speeds, etc. Another factor which has a significant effect on compressor operating efficiency relates to the extent to which the compressor is loaded. In fully-loaded operation, the maximum output of the compressor is achieved, with a resultant full load on the compressor engine. However, natural gas compressor flow demands can vary considerably, and typically depend on downstream demand factors and conditions.
Controlling compressor flow is often accomplished by partially xe2x80x9cunloadingxe2x80x9d a compressor whereby each compressor stroke produces a reduced gas flow as compared to fully-loaded operation. Reduced gas flow generally corresponds to reduced work performed by the compressor engine, whereby fuel savings and greater efficiency can be achieved. Although compressor output could be varied by changing the speed of the driving engine, this approach is often impractical because the engines are designed to operate at constant speeds for maximum fuel efficiency and minimum emissions. Thus, compressor output control must normally be accomplished using other means.
A compressor can be partially unloaded and its output reduced by increasing the clearance volume. Clearance pockets or clearance bottles connected to the compressor cylinder via an unloader valve are often provided for this purpose. The clearance pocket may be built into the cylinder head or installed outboard of a respective suction or discharge valve.
Owsley et al., U.S. Pat. No. 4,737,080, which is incorporated herein by reference, discloses a valve assembly having valve members which are controlled by means of a pilot valve. The valve assembly is mounted in a respective suction or discharge valve pocket such that the valve members serve as intake or discharge valves for the compressor and also provide means for unloading the compressor. If, for example, the valve assembly is installed in the suction line of the compressor, the heads of the valve members are placed in communication with the respective suction line. A source of low pressure (such as the atmosphere) is selectively applied to the stems of the valve members through the pilot valve to create a pressure differential across the valve members which results in the valve members being forced into an open condition and held open. With the valve members thus held open, the compressor cylinder is placed in continuous communication with the suction line, fully unloading the compressor.
An alternative embodiment of the valve assembly of Owsley et al. adds a clearance bottle and an annular secondary valve assembly to the device. The clearance bottle is positioned over the valve assembly such that the valve members previously described act as primary valve members which control flow between the cylinder and the clearance bottle. The secondary valve assembly includes secondary valve members which control flow between the clearance bottle and the suction line. The primary and secondary valve members are all selectively controlled by the pilot valve, which is a three way valve.
In a fully-loaded operating condition the secondary valve members allow flow between the suction line and the primary valve members. The primary valve members are allowed to operate as the suction valves (i.e, to close on the compression stroke and open on the suction stroke of the compressor). In a fully-unloaded condition, both the primary and secondary valves are held open, thereby placing the compressor cylinder in continuous communication with the suction line. In the third possible position of the pilot valve, the primary valve members are held open and the secondary valve members are allowed to function as the suction valves. This, in effect, adds the entire volume of the clearance bottle to the clearance volume of the compressor cylinder and thereby partially unloads the compressor.
A problem with this type of clearance bottle unloader system is that the operation of the unloader is simply and on/off selection, meaning that the bottle is either in continuous communication with the compressor cylinder, or it remains out of communication with the compressor cylinder. The device has no capability for allowing partial use of the clearance bottle between the open and closed conditions.
Sperry, U.S. Pat. No. 5,695,325, which is incorporated herein by reference, discloses an unloader system wherein the compressor may be unloaded in small increments during operation by rotating a valve guard mounting the valve members in synchronization with the compressor crankshaft. This is accomplished using a stepper motor keyed to the compressor""s crankshaft position to actuate a radial unloader valve assembly. While this arrangement does allow the compressor to be loaded and unloaded incrementally, the mechanism is rather complex and not suited for every compressor unloading application.
The present invention relates to pneumatically loading and unloading a reciprocating compressor in a smooth, stepless manner. This is accomplished by using a controlled pressure to hold the unloader valve members closed until the pressure in the compressor cylinder reaches the desired level. By adjusting the set point of a pressure regulator, the effective use of any shape and size of clearance cavity can be smoothly varied from zero impact to full impact.
Heretofore there has not been a compressor unloader system available with the advantages and features of the present invention.
In the practice of the present invention, an unloader system is provided for a reciprocating gas compressor having a cylinder, a piston reciprocally mounted in the cylinder, a suction line, a discharge line, a suction valve assembly and a discharge valve assembly for selectively communicating the suction and discharge lines respectively with the compressor cylinder. The unloader system includes a clearance cavity in communication with the compressor cylinder though a passageway and an unloader valve assembly having one or more valve members moveable between open and closed positions and controlling flow through the passageway. The valve members each have opposed first and second ends with the first ends being acted on by pressure in the compressor cylinder. The cylinder pressure produces a first force which acts to urge the valve members toward their open positions.
A conduit communicates the second ends of the valve members with a pressure regulator. The regulator is also in communication with a pressure source. Pressure from the pressure source is selectively varied by the pressure regulator to create a control pressure which acts on the second ends of the valve members to produce a second force which acts in opposition to the first force and urges the valve members toward their closed positions. The valve members open when the first force exceeds the second force and close when the second force exceeds the first force.